When a fluid-containing housing, for example the housing of a pump or turbine, is penetrated by a rotating shaft, a mechanical seal is often used to minimize leakage of a process fluid from the interior of the housing. End face mechanical seals, also referred to as mechanical face seals, are commonly used for this purpose.
Mechanical seals rely on maintaining a closely spaced relationship between the flat and parallel surfaces of a stationary, “stator” seal face and a rotating “rotor” seal face. This generally requires that a lubricant or other “cushioning substance” occupy the small gap between the stationary and rotating seal faces, so as to minimize frictional heating and premature wear.
In circumstances where the process fluid is non-lubricating and/or hazardous, a separate, pressurized lubrication system is sometimes provided that introduces a liquid lubricant into the seal. The lubricant is applied with sufficient pressure to cause it to slowly leak through the seal into the process, thereby preventing any escape of the process fluid through the seal into the surrounding environment. While this approach can be highly effective, it is also complex and expensive.
In situations where the process fluid is a liquid with suitable lubricating properties, the process liquid itself is often used as the seal lubricant by allowing the process liquid to enter the seal and slowly leak past the seal faces. This approach has the advantage of not requiring a separate, complex and costly lubrication system. However, this approach has the disadvantage that small amounts of process fluid is allowed to leak past the seal and into the surrounding environment, which may be unacceptable in cases where the process fluid is hazardous or otherwise undesirable.
Under such circumstances, one approach is to provide a second, “dry” mechanical seal in tandem with the primary seal that will intercept the slowly leaking process fluid and allow it to be recaptured without escaping into the environment. Such dry mechanical seals may be “lift-off” seals, whereby the rotor and/or stator seal faces include lift-off features that create a pressurized air cushion during operation that fills the small gap between the two seal faces, thereby minimizing friction, wear, and heating. An example of this arrangement is shown in FIG. 1, which includes a primary mechanical seal 100 that is lubricated by the process liquid, arranged in tandem with a secondary, dry mechanical seal 102 that is lubricated by a cushion of air. This approach has the added advantage that the secondary, “dry” seal 102 can provide temporary backup sealing in the event that the primary seal 100 suddenly fails, giving the operator of the equipment enough time to invoke immediate shutdown and isolation procedures during such an event, without allowing a major release of process fluid to the atmosphere.
In some applications, such as centrifugal pumps used in some petroleum refineries, the pumps are often provided in pairs, and are operated such that one pump is in operation while an adjacent pump of the same size and type is maintained in a standby mode, typically with its shaft spinning at a very low speed. Under such circumstances, the rotation speed while in stand-by mode may not be sufficient to enable a dry mechanical seal 102 such as the one illustrated in FIG. 1 to generate a lubricating air cushion, thereby causing the seal to operate in stand-by mode with the stator and rotor faces in direct mechanical contact. This can lead to damage of the lift-off features on the rotor and stator seal faces, such that they are no longer able to generate a lift-off cushion even when they are subsequently switched to normal operating mode with high shaft operating speeds. As a result, the seal faces may remain in contact when they are subsequently operated at high rotational speeds, which will cause the faces to overheat and wear at a rapid rate.
What is needed, therefore, is a dry running mechanical seal design that is able to maintain the faces in direct contact for extended periods of time at both low and high operating speeds, without requiring a lift-off feature, and without undue heating or wear of the seal faces when the seal is operated at high shaft speeds.